1. Field of the Invention
The present invention relates to mineral concentrators which concentrate specified, relatively dense minerals, particularly gold, from a raw material mixture containing both the concentrate and less dense particles of classified sizes. The raw material is immersed in water and moved by the water and force of gravity over a separation surface which is in motion. More particularly, the present invention relates to oscillating table concentrators of the type in which a preprocessed feed material or raw material is continuously fed to a feed end of the concentrator and is continuously discharged from a discharge end of the concentrator, segregating denser minerals in the process.
2. Description of the Prior Art
Particularly in connection with gold recovery, though useful in recovering other minerals as well, it is known to wash a raw material mixture including relatively denser particles over a moving, separation surface. Waste materials, including tailings and middlings, flow with wash water to one portion of a concentrator apparatus, while the mineral concentrate is collected and moved to another part of the concentrator apparatus as a result of a relatively higher density. It is common practice to use water on the separation surface to help stratify and move the raw material along various flow paths established according to the different mineral specific gravities constituting the raw material.
The simplest concentrator is a gold pan wherein a shaking and swirling action is imparted to the raw material and water held in a pan. Lighter materials are systematically washed out of the pan, leaving the denser minerals and gold at the bottom of the pan. Gold panning is an art that requires patience and skill in order to separate gold from relatively dense mineral concentrates. It is also useful in processing only small volumes of raw material.
Another type of concentrator having intermittent feed and discharge, as does panning, is the cradle or rocker. An elevated screen receives the raw material to be processed and water is poured onto the raw material washing the fine material through the screen. The coarse material of a greater size is removed from the screen and thrown away. The cradle is rocked about a longitudinal axis during the washing process and any fine material washed through the screen strikes a canvas apron, where some of the gold or mineral concentrate is held for recovery. From the apron, the raw material is washed over a series of riffles transverse to the longitudinal axis, where additional gold settles out, the waste material exiting at a tail end of the cradle. Again, gold is difficult to separate from dense mineral concentrates and the volume processed is quite small.
Jerking and bumping tables have a planar separation surface or deck that is oscillated along a longitudinal axis. Both jerking and bumping tables operate on principles relating to the momentum of the particles of raw material moved by wash water across a single sloping surface comprising a deck. The oscillation of a jerking table is adjusted so that a relatively higher velocity is imparted in one oscillation direction than in the other direction. The result of the different velocities is a net particle movement in the direction of lesser velocity. The bumping table has identical velocities in either direction along the longitudinal axis, but an intermittent force is applied directly to the deck in a direction toward a head or feed end. The momentum of the raw materials carries them toward the tail or discharge end.
It is also common practice to separate gold from concentrate produced by prior art devices by amalgamation by mercury. The resultant amalgam is then retorted to separate the gold. The recovery percentage of fine-sized gold is low in all these processes.
The jerking and bumping tables continuously receive raw material at the feed end and continuously discharge raw material and concentrate along particle flow paths. The entire surface or deck is laterally inclined downwardly a few degrees with respect to a horizontal plane so that the water flows across the deck at about right angles to the direction of oscillation. These tables are usually level with reference to a longitudinal attitude, but might have a slight positive or negative inclination, from the feed end to the discharge end. The separation surface includes a series of longitudinally extending narrow riffles. Typically, the riffles are raised and may be established by tacking tapered cleats on the surface of the deck or may be formed of rubber, metal or other substances. In either event, the riffles formed on the surface are relatively narrow and are all of equal cross sectional dimensions at a given position along the length of the deck. Most riffles extend substantially the length of the deck and are numerous in number. Commonly, a series of raised or recessed riffles taper along their length as the discharge end is approached. The riffles terminate on a diagonal line, relative to a longitudinal axis of the deck, defined by connecting ends of the riffles.
It is known to oscillate the deck by a single apparatus combining eccentrics, toggles and springs. Eccentrics alone can be used as can cams, springs and bumping posts in combination. However the deck is shaken, in a jerking table the velocity of the deck toward the feed end of the deck must be more rapid than toward the discharge end of the deck. As has been stated, this relative velocity difference can be established in a bumping table by an intermittent force applied to a deck otherwise oscillating in each direction at the same velocity.
This relative velocity difference tends to move the raw material particles fed onto the deck along distinct flow paths, which paths are in direct relation to the particle size and density. The oscillation is in line with the orientation of the riffles, i.e., along the longitudinal axis. The denser minerals, such as gold, tend to lie in the riffles and move toward the discharge end of the table. The lighter, less dense middlings and tailings, or waste raw material, tend to be displaced from the riffles by denser minerals and washed out of the riffles down the sloping surface of the deck to a side edge of the deck, rather than being moved to the discharge end.
On a rectangular table with longitudinally extending riffles, there will be a separation line between flow paths essentially on the diagonal, separating the deck into two areas. The area of the deck nearest the feed end will discharge waste material while the remaining area of the deck and the riffles extending therealong will tend to carry concentrate to the concentrate or discharge end. It is known to establish the separation line by a flexible bend in the deck itself.
Of the several types of oscillating tables, the Wilfley table is the oldest and most widely used. It requires a substantial foundation or support in order to resist the stress of the vibration induced by oscillating the table at 240 cycles per minute. Motion is imparted to the table deck by a pitman driven by a crank and oscillating the deck through two different toggles. A spring damps the entire system. The frequency of oscillation can run from as low as 150 cycles per minute to as high as 290 cycles per minute, the average being about 240 cycles per minute. The Wilfley table also includes a tilting frame for varying the slope of the deck to meet variations in the raw materials. The riffles, either of the recessed or raised cleat type, are tapered lengthwise and terminate at the concentrate end. The riffles are known to be square, sawtooth, or V-shaped in cross section.
A modification to the Wilfley table, known as the Butchart table, has a different orientation and design for the continuous riffles. Three distinct zones are defined upon the deck. A first stratification zone is similar to Wilfley and includes the deepest part of the riffles. The first zone is longitudinally adjacent a feed box at the feed end of the deck. The second zone is a cleaning zone which consists of an area of riffles bent up the sloping surface towards the water feed side of the deck, defining an angle with a longitudinal axis of the deck which is out of longitudinal alignment with the riffles in the first stratification zone. The deflection of these riffles in the cleaning zone, and in combination with the slope of the deck, produces an uphill flow of material, reversing the normal flow of material along the riffles. The last zone, called the discharge zone, is riffled like the first zone and ends in an unriffled portion like a Wilfley table.
In the Butchart table, stratification occurs quite early when water washes the raw material over the riffles and the waste in channels or troughs nearer the feed end and at the downhill side of the deck. The cleaning zone imparts a different kind of action to the riffles, a "side shake" similar to that of a vanner resulting in further stratification and cleaning of the raw material in this zone of riffles.
Another type of table has come to be known as the Deister table. The Deister table differs from the Wilfley table primarily in that it includes a deck that is not rectangular but rather more of a parallelogram shape.